1. Field of the Invention
The present invention provides coating compositions suitable as spin finishes for ceramic fibers and as coatings on ceramic and metal matrix-ceramic composites. More particularly, the coatings are especially suitable for compatibilizing ceramic superconducting fibers with a metal matrix, and especially as applied as a spin finish for superconducting ceramic fibers, both in the green, unfired stated and in the sintered state.
2. State of the Art
The art of spin finishes has been well-developed over the past decades, especially in the area of textile fibers. A spun fiber is typically pulled across rollers and similar devices, and it may be subjected to various treatments, before being wound onto a spool. Generally, a spin finish is applied to the as-spun fiber to reduce the friction between the fiber and the various fixed routing devices (e.g., rollers). A spin finish may also be applied to prevent fibers, once wound, from adhering to each other. Other spin finishes find importance in chemical or physiochemical treatments of the fiber. For example, a spin finish may be deposited on the fiber to improve absorption and adherence of dye in a later dying step.
On the other hand, a spin finish or other coating may be applied to a fiber to prevent reaction. For example, carbon fibers are typically provided with a coating because they readily burn. Coatings for carbon fibers are generally provided by electrodeposition. Typical coatings for carbon fibers include copper, nickel, teflon, and the like. Nickel coated carbon fibers have been used for EMI (electromagnetic interference) shielding by mixing pellets of the coated fiber with a conventional engineering plastic (a "pellet" is formed from a tow of 3,000 to 12,000 filaments which is chopped into staple lengths of about 0.25".) See, e.g., B. A. Luxon and M. V. Murthy, "Metal Coated Graphite Fibers for conductive Composites," reprinted from Proc. of the Soc. Plast. Eng. 44th Annual Technical Conference & Exhibit (1986).
The recently devised ceramic superconductors present a problem yet to be addressed. It is acknowledged in the art that a superconducting ceramic fiber, in order to function as a conductor, will have to be in intimate contact with a metal conductor in case of a quench; i.e., since the present superconducting ceramic fibers are superconducting only at reduced temperatures, in the event of a major thermal disturbance (e.g., in the situation where the refrigerant system were to fail), the entire electrical current would have to be shunted to a "normal," metallic conductor. However, ceramic-to-metal bonding is not easy to accomplish, even in metal-ceramic composites. Accordingly, to use ceramic superconductors as electrical conductors, it would be advantageous to improve the ceramic-to-metal bonding characteristics.
It is also suggested that these superconducting ceramics would be most advantageously utilized in the form of a wire. Recent investigators of the superconducting ceramics have employed a variety of techniques to produce superconducting ceramic fibers. For example, Jin et al., Appl. Phys. Lett., vol. 51, no. 3, pp. 203-4 (20 July 1987), describes the production of wires by compacting finely pulverized YCBCO is YBa.sub.2 Cu.sub.3 O.sub.7-x, also known as the "123" superconductor) powder into a metal tube and drawing the filled tube into a wire. These wires are then sintered, but this high temperature sintering depletes the YBCO of oxygen; superconducting YBCO is of a stoichiometry that is hyperoxygenated (i.e., x is less than 0.15; preferably x is less than 0.10). Accordingly, subsequent processing, further hampered by the outer metal tube, is required to readjust the stoichiometry.
Additional and related methods are described by Jin et al. in Appl. Phys. Lett., vol. 51, no. 12, pp. 943-945 (21 Sep. 1988)(submitted 2 July 1987; available in the Scientific Library of the U.S. Patent and Trademark Office as of 13 Aug. 1987 under catalog number 0175). They describe melt-processing, by "drawing" or "spinning" as defined therein, of a molten pressed compact of fine YBCO powder. Again, the high temperature processing conditions result in an oxygen depletion from the necessary superconducting stoichiometry. Thus, the methods of Jin et al. are difficult to control in production, in part because the high temperature processing of YBCO previously of the desired stoichiometry necessitates a subsequent adjustment of the oxygen stoichiometry to achieve a superconducting ceramic.
Additionally, for a flux-jump stable multifilamentary conductor, the smaller the fiber diameter the more stable the conductor. A flux jump represents a local quenching of the superconductor, a collapse of the associated field, and the release of energy; a superconductor can experience its first flux jump when the stored energy of the field released into the superconductor is just sufficient to raise its temperature above T.sub.c (superconducting transition temperature). The thickness of a superconductor meters the quantity of flux associated with a jump (the liberated heat being proportional to the square of the thickness), and thus the likelihood of a flux-jumping event is reduced by decreasing the thickness (i.e., making finer filaments). See, e.g., E. W. Collins, APPLIED SUPERCONDUCTIVITY, Metallurgy, and Physics of Titanium Alloys, vol. 2, chpt. 25 (New York, Plenum Press: 1986).